Context

From Linux-VServer

context is the unique integer, used to identify each guest instance. You may also come across it under the name xid or client id.
In the context of guest creation, you may also leave it away, as the creating program or script can enumerate them (pick the next available xid) automatically, but for presumably demonstrating manual control over the xid, it is shown here. resumehelp.ws/writers.php
There may a hardcoded upper limit to xid at 49152